With the increased focus on vehicle economy, particularly vehicle fuel economy, automotive manufacturers are turning to smaller, lighter vehicles and unique vehicle powertrains to boost efficiency. Recirculated exhaust gas (“EGR”) is utilized in most conventional internal combustion engines to assist in the reduction of throttling losses at low loads, and to improve knock tolerance and reduce the level of oxides of nitrogen (“NOx”) in the exhaust gas at high engine loads. EGR is especially important as an emissions reducer in internal combustion engines that run lean of stoichiometry and thereby are prone to emitting higher levels of NOx emissions.
One proposition that has been considered in the construction of internal combustion engine systems is to utilize one of a plurality of cylinders as a dedicated EGR source. For example, in a four cylinder engine, the entire supply of exhaust gas produced in one of the cylinders is transferred to the intake ports of the other three cylinders as EGR. The EGR-producing cylinder may be operated at customized levels of air and fuel; as may be determined by an engine controller that is in signal communication with various engine, vehicle and exhaust system sensors. Since the exhaust gas from the EGR-producing cylinder is to be re-circulated before being released to the atmosphere, the customized air and fuel levels in the EGR-producing cylinder may be optimized to achieve selected goals such as engine efficiency, power, and operability.
Since exhaust gas produced by the remaining two cylinders is to be released to the atmosphere following treatment in an exhaust gas treatment system, the air and fuel mixtures of these remaining cylinders are operated so as to meet emission standards. Fortuitously, these remaining cylinders enjoy benefits associated with ingestion of EGR from the EGR-producing cylinder. These benefits include reduced combustion temperatures and associated levels of NOx, allowing richer levels of EGR in the remaining cylinders with increased levels of hydrogen, thereby improving knock resistance, fuel consumption and combustion stability, while still allowing stoichiometric gas to be maintained in the exhaust gas treatment system for compatibility with the catalytic treatment devices.
A disadvantage to this type of internal combustion engine system is that an internal combustion engine that uses only a single cylinder as the dedicated EGR cylinder may not uniformly deliver EGR volumes to the remaining cylinders. For example, the cylinder event following the dedicated EGR cylinder event may be prone to receive more EGR diluent than the subsequently firing cylinders. This variation in cylinder makeup (i.e. combustion air, fuel and EGR diluent) can result in uneven combustion performance that is difficult to control over a broad range of operating conditions.
To at least partially address these disadvantages, a number of configurations are being studied, including configurations wherein more than one in four cylinders operates as a dedicated EGR cylinder or where a dedicated EGR cylinder produces more than a single volume of exhaust gas for every four volumes of exhaust gas produced by other cylinders. To enable such configurations, it would be advantageous to have a crankshaft that can facilitate improved distribution of EGR among non-EGR cylinders. It would also be advantageous to have a crankshaft that can enable cylinders displacements that differ between the EGR and non-EGR cylinders.